1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor suitable for a semiconductor laser of a short wavelength capable of realizing a high resolution of an image, an image forming apparatus, and a process cartridge attachable/detachable to/from an electrophotographic apparatus main body.
2. Description of the Related Art
In recent years, organic photoconductive materials have been widely used more frequently in electrophotographic photoreceptors generally by virtue of their advanced development, compared to inorganic photoconductive materials that have been conventionally used. This is because electrophotographic photoreceptors using an organic photoconductive material have many advantages in terms of toxicity, cost, flexibility in material design and the like, over the inorganic photoconductive materials although it has some problems in terms of sensitivity, durability and environmental stability.
As structures of electrophotographic photoreceptors having been put into practical use at present, there are proposed layered-type or distributive-type function separated type photoreceptors in which a charge (electron, positive hole) generating function by a photoconductive material, and a charge transporting function for transporting the generated charge by electric field applied on the electrophotographic photoreceptor are respectively assigned to separate substances.
Such a function separated type photoreceptor accepts a wide range of substances for respective functions, and hence is able to provide a photoreceptor realizing high performance in electrophotographic characteristics such as charge characteristics, sensitivity, a residual potential, repeating characteristics, printing resistance and so on, by combining best substances.
Furthermore, since it can be produced by applying a photosensitive layer on a conductive supporting member, it is possible to provide a photoreceptor with very high productivity and a low cost, and to freely control a photosensitive wavelength region and photosensitivity by selecting an appropriate charge generating material.
Furthermore, owing to improvement in performance of electrophotographic photoreceptors using an organic photoconductive material to overcome conventional problematic points in characteristics, for example, ability of designing a photoreceptor having excellent abrasion resistance by appropriately selecting a binder resin to be contained in the charge transporting layer, organic photoconductive materials have been used more often as compared to inorganic photoconductive materials.
As an electrophotographic apparatus using a laser beam as an optical source of light exposure, a laser printer can be recited as a representative example, however, recent advanced digitalization has made common to use a laser beam as an optical source of light exposure in copying machines.
As a laser beam mainly used as an optical source for light exposure, a semiconductor laser which is low in cost, small in consumed energy, light in weight and small in size has been brought into practical use, and a typical laser has an emission wavelength in a near-infrared region around 800 nm from the viewpoints of stability in an emission wavelength and output and life time.
This is because a laser beam having an emission wavelength in a short wavelength region has not been put into practical use due to technical problems. In light of this, as a charge generating material used in an electrophotographic apparatus using a laser beam as an optical source for light exposure, a layered-type photoreceptor in which an organic compound having light absorption and light sensitivity in a long wavelength region, in particular, phthalocyanine pigment is contained in a charge generating material has been developed.
On the other hand, in order to improve image quality of output image of an electrophotographic apparatus, consideration is made to increase the resolution of the image. Some measures are conceivable to achieve images of a high recording density and a high resolution, and as an optical measure, it can be recited to increase the writing density by narrowing down the spot diameter of the laser beam.
For achieving this, a focal distance of the using lens may be shortened, however, it was found that in addition to the difficulty in designing an optical system, in the laser having an emission wavelength in a near-infrared region of around 800 nm, sharpness of spot contour is difficult to be obtained even when the beam diameter is narrowed down by operation of the optical system. This is attributable to a diffraction limit of a laser beam, which is inevitable phenomenon.
However, when a spot diameter of a laser converged on a surface of a photoreceptor is taken as D, the relation represented by:D=1.22λ/NA (λrepresents a wavelength of a laser beam, and NA represents the number of lens apertures) is satisfied.
From this formula, it can be found that since spot diameter D is in a proportion to an emission wavelength of a laser beam, a laser with a shorter emission wavelength may be used to decrease the spot diameter D. Also, Japanese Patent Application Laid-Open Publication No. 5-19598 proposes an electrophotographic apparatus using a short wavelength laser.
In view of the above, it is recently conceived to use a blue (violet) semiconductor laser of a short wavelength that is getting into practical use for DVD, as a light exposure optical source (writing optical source) of an electrophotographic apparatus. When a blue (violet) semiconductor laser beam (380 to 500 nm) having about one third to half of an emission wavelength compared to a conventional semiconductor laser beam in a near-infrared region is used as a light exposure optical source, it is possible to make the beam spot diameter very small while keeping the sharpness of the contour as shown by the above formula. Therefore, it provides a very effective measure for realizing super-fine image quality.
By using a blue (violet) semiconductor laser beam as an optical source for light exposure in the manner as described above, it is possible to irradiate the electrophotographic photoreceptor with a beam spot diameter of about 40 μm or less while keeping the sharpness of the contour.
Hence, in an electrophotographic apparatus in which a blue (violet) semiconductor laser beam is used as an optical source and a beam spot diameter is reduced, an electrophotographic photoreceptor having a certain degree or higher sensitivity to light irradiation of an image light exposure apparatus is naturally needed.
Further, in order to use the light emitted to the electrophotographic photoreceptor effectively, it is requested to have high spectral sensitivity in the wavelength region of the optical source. Further, to utilize the small beam spot diameter more efficiently, a higher resolution is realized by reducing the film thickness of the charge transporting layer.
However, the number of electrophotographic photoreceptors having high spectral sensitivity in the wavelength region of the optical source is very small. A variety of researches are now underwent, taking note of organic photoreceptors having various advantages including excellent environmental compatibility, easiness of production and handling, and low cost.
For example, as for azo pigments intended for a blue (violet) semiconductor laser, Japanese Patent Application Laid-Open Publication No. 10-239956 discloses an exemplary embodiment using an anthraquinone-based azo pigment, and Japanese Patent Application Laid-Open Publication No. 2000-105478 discloses an exemplary embodiment using an azo pigment having various couplers.
However, in any of these cases, sufficient sensitivity is not achieved for a blue (violet) semiconductor laser.
Further, in order to improve the image quality level by using a blue (violet) semiconductor laser as an optical source and reducing a beam spot diameter, it is generally requested to reduce a film thickness of the photosensitive layer. However, it is also requested to improve mechanical printing resistance for reducing the film thickness of the photosensitive layer while keeping conventional life time. For achieving this, the measure of increasing the content of binder resin and the like is taken. However, when the content of binder resin increases in comparison with the charge transporting material, the problem arises that the electric characteristics such as sensitivity and light response deteriorate.